Salt bath furnace



May 8, 1956 A. L. BOEGEHOLD SALT BATH FURNACE Filed Jan. 19, 1952 Q4212E g/ 11% ATTORNEYS \\IW 4 N W 3 k H H f w f E5: W .QYM 2 0,, I n a j 5\w\ United States Patent SALT BATH FURNACE Alfred L. Boegehold, Detroit,Mich, assiguor to General Motors Corporation, Detroit, Mich., acorporation of Delaware Application January 19, 1952, Serial No. 267,259

13 Claims. (Cl. 118-429) The present invention relates to furnaces and,more particularly, to furnaces adapted to dipping processes for cleaningand for coating metals with aluminum or other metals.

As is well known in the art, in order to obtain a good bond between aferrous or other base metal and a coating metal such as aluminum, it isessential that the surface of the base metal be absolutely clean at thetime the coating metal is applied. This is advantageously accomplishedby cleaning the base metal in a suitable salt bath. In order to precludeoxidation of the surface of the base metal prior to the application ofthe coating material, it is desirable to utilize a dipping processwherein the cleaning treatment and the coating are accomplished in thesame bath so that the once cleaned metal is not subjected to air inbetween the cleaning and the coating steps. For such a process a bathhaving a lower layer of molten aluminum and an upper layer of moltensalt may be used so that the metal article to be coated can be passedthrough the salt layer for cleaning and for heating and then into themolten aluminum layer for coating. As is disclosed in U. S. Patents2,544,670, 2,544,671 and 2,569,097, a salt bath which has been found tobe particularly adaptable to the cleaning of ferrous and other basemetals prior to metal coating is one containing a mixture of metalfluorides, together with other halides and maintained in contact withmolten aluminum for activation of the salt.

While such a molten fluoride containing salt in contact with moltenaluminum has many advantages as a preheating and cleaning agent formetal coating processes, it is disadvantageous in that it is extremelyreactive and will Within a relatively short period corrode the usualfurnace linings. Another difliculty which is encountered in coatingprocesses utilizing a bath having contacting fluoride containing moltensalt and molten aluminum layers is that the proper temperature controlof the salt and aluminum layers is diiiicult.

It is an object of this invention to provide a furnace having increasedresistance to corrosion and particularly adapted to metal coating andcleaning processes wherein a fluoride containing salt in contact withmolten aluminum is used. Another object is to provide improved heatingand heat control means for furnaces used in metal coating and cleaningprocesses.

These objects are carried out in accordance with this invention by theprovision of a silicon carbide lining at least on those portions of thefurnace which are subjected to the corrosive action of the metalfluoride salts and by the provision of a resistance heating means in themolten salt layer with or without additional induction heating means inthe molten aluminum layer.

Other objects and advantages of my invention will be apparent from thefollowing description and from the drawings in which Figure 1 is avertical section of one embodiment of my invention; Figure 2 is a viewof the embodiment shown in Figure 1 looking in the direction of thearrows 22 of Figure 1, with parts broken away and in section; Figure 3is a vertical sectional View, in reduced scale, of another embodiment ofmy invention, and Figure 4 is a vertical sectional view of still anotherembodiment of my invention.

In the embodiment shown in Figures 1 and 2 the furnace consists of atank 5 of rectangular cross-section and having a liner 6 surrounded by aheat insulating material 7 such as brick of some suitable ceramic backedup by packed asbestos. This entire structure rests on a support such asthe metal beam structure 8.

When in operation the tank 5 contains an upper molten salt layer 9 whichconsists of a mixture of metal fluorides and other halides, and a lowermolten aluminum layer 10. By the use of the term aluminum herein ismeans not only pure aluminum but also aluminum base alloys.

The dimensions of the tank will of course depend upon the depth of saltand metal layers to be used and on the size of the metal articles to bealuminized, etc. The upper portion 11 of the tank liner 6 consists of amaterial which is highly resistant to the corrosive action of moltenmetal fluorides and other halides. Silicon carbide is particularlysuitable for this purpose because of its high resistance to attack bythe molten fluorides and other halides. The bottom portion 12 in contactwith the molten aluminum layer is made of any suitable refractory suchas alumina or graphite. Monolithic linings of aluminum silicate bondedby sodium silicate or of silicate bonded silica are, for example,satisfactory. it will be noted that in this embodiment the upper orsilicon carbide portion 11 of the liner consists of an insert or surfacelayer set into the backing portion of the liner 6, the backing portionbeing of the same material as the lower portion 12. It is understood, ofcourst, that the entire tank lining could be of silicon carbide.However, in the preferred embodiment a silicon carbide liner is usedonly on those portions which are subjected to the molten salt, chieflybecause of the savings in cost. Also, it has been found that amonolithic alumina or silica liner is more durable than is siliconcarbide in those portions contacted by the molten alumina. It will benoted that the silicon carbide portion 11 of the tank lining extendsslightly below the depth of the salt layer. This struc ture is desirablebecause of the slight differences in the heights of the aluminum layerwhich are encountered in the operation of the furnace. The extension 13of the silicon carbide liner below the interface of the two layers Willfurther preclude the possibility of the molten salt coming into contactwith the lower liner portion 12 with consequent erosion.

Extending into the salt layer from above the tank 5 and adjacent a sidewall are a series of electrodes 14 which are electrically connected byconductors such as bus bars 15 to a source of electrical energy orcurrent generally indicated at 16. The electrodes and electricalconnectors can be supported in any suitable manner as, for example, bythe electrical insulators 17 which secure the bus bars 15 to which theelectrodes 14 are fixed, to the outer furnace wall. An electrode spacerbar 18 made of some heat resistant electrical insulating material mayalso be used to help maintain the electrodes in position. As can best beseen in Figure 2, the electrodes 14 are positioned adjacent one side ofthe tank 5 in order to allow for greater working space. With thisarrangement metal articles can be dipped without interference from thepositioning of the electrodes. It is to be understood, of course, thatelectrodes could be positioned on more than one side of the tank or forthat matter toward the center of the tank if such were necessary inorder to supply sufficient heat to a large furnace. The electrodes 14are of a length suflicient to extend to a depth above the bottom of thesalt layer. Since shorting will result if the electrodes contact themetal layer, it is of course necessary to. position the electrodes sothat there will be no contact with the bottom layer even after theimmersion of the metal work pieces into the metal with a resulting riseof the level'of that layer. The most desirable length of the electrodesto be us-ed will depend, therefore, upon the depth of the salt layer andalso upon the volume of the work to be immersed at any one time into themetal layer.

The salt layer 9 is maintained molten by the resistance heating whichresults when current is passed through the salt by means of the seriesof electrodes 14. The aluminum layer 10 is maintained molten by contactwith the molten salt layer. I have found that aluminum layers up to 12inches in depth can be melted and maintained molten under a molten metalhalide layer.

In order to effect a better heat transfer from the salt layer to thealuminum layer it is desirable to utilize a stirring mechanism toagitate and drive the hot molten salt down to the salt-aluminuminterface. The stirring mechanism used in the modification shown byFigure 1 consists of a pair of impellers 19, one impeller being locatedon each side of the tank and adjacent a'wall. Each impeller consists ofa series of impeller blades'2tl mounted on a shaft 21 which is rotatedby the chain or belt-driven pulley 22. The chain or belt 23: is in turndriven by an electric motor 24 which is mounted on an outside wall ofthe furnace away from the heat. The impellers may be secured to thefurnace by bolts (not shown) or by any other suitable means.

It will be seen that, by rotating the impellers 19, the hot salt will beagitated and driven down against the aluminum layer, thus resulting in agreater heat exchange from the salt to the aluminum. It is to beunderstood that any suitable impeller means can be used, the specificexample merely serving for purposes of illustration.

Supplementary to the heating and stirring means described above is thatwhich results from the feeding of the work into the furnace. It is, forexample, often the practice to preheatthe metal to be coated prior toimmersion into the furnace. In this instance the heat thereby suppliedwill aid in maintaining the temperature of the salt and aluminum layers.Likewise, when work is fed rapidly through the layers, as for examplewhere sheet metal or individual work pieces are driven through thefurnace by means of a conveyor system, the agitation which results willsupplement that obtained by the impellers. While it is not alwaysessential to use impellers, it is usually desirable to do so, especiallyin those cases where a deep aluminum layer is used and where the work isnot fed at a rate sufficient to cause considerable agitation.

I wish to point out that any suitable conveyor system can he used inconjunction with the furnace of this invention. Wire, sheet metal, orindividual work pieces, can be fed into the furnace either by anautomatic conveyor or manually.

Generally it is advantageous to accomplish both the metal cleaning andthe metal coating steps in the same furnace. However, it is of coursepossible to effect each operation in a separate furnace. If this latterprocedure is used, then the furnaces of this invention will serve wellfor the cleaning step wherein a molten fluoride containing salt incontact with molten aluminum for activation thereof is used as the fluxor cleaning agent. In this instancea shallower layer of aluminum may ofcourse be used than if the furnace is used for both the cleaning and thecoating steps.

Figure 3 shows a modification wherein a lesser amount of aluminum isutilized to obtain the same layer depth, and consists of a tank 26having a silicon carbide liner 27 surrounded by insulating material 28such as asbestos and/or ceramic brick and supported by metal beams orother suitable structure 29. A second and smaller tank 30 having siliconcarbide walls 31 is submerged within the tank 26. The submerged tank 30is constructed with a large base portion 32 for purposes of strength andstability. In this construction the molten aluminum 33 is contained inthe submerged tank 3t and is maintained molten by heat transfer from themolten salt 34 in the tank 26. In the particular embodiment shown thesubmerged tank 31 is positioned in the center lengthwise of the tank 26and toward one side; however, it may also be positioned in the exactcenter of the tank or, alternatively, it may be positioned against theside of tank 26 so that it has one wall in common with the larger tank.In the embodiment shown the small tank 30 is completely separate fromthe large tank 26 in which it is submerged and thus can be removed formaintenance without disturbing the large tank.

The walls 31 of the submerged tank 30 may be made entirely of siliconcarbide or else may be made of silicon carbide only in those positionssubjected to molten salt and of another refractory such as aluminia,silica or graphite in those portions in contact with the moltenaluminum.

Electrodes 35 extend into the salt layer 34 and thereby maintain thesalt molten by way of the resistance heat ing which results when currentis passed. Suitable electrical connectors and a source of electricalenergy the same or similar to those shown in Figure 2 are of course usedin the embodiment shown in Figure 3. Impellers such as those shown inFigures 1 and 2 may be used in the furnace shown in Figure 3; however,in this particular embodiment there is less need for agitation of thesalt layer because of the larger volume of the salt in comparison tothat of the aluminum and because of the fact that the molten salt morenearly surrounds the aluminum layer. Also, the excellent heatconductivity of the silicon carbide walls 31 effect efficient heattransfer from the salt to the aluminum.

When the aluminum is heated by heat transfer from the salt layer, thealuminum is of course always colder than the salt. With a 12-inch deeplayer of aluminum and the salt at 1300 F., there may be, for example, a40 to 60 F. difference in the temperature between the salt and aluminumlayers even when the salt is circulated by impellers. As the requiredaluminum temperature becomes greater, so also the temperaturedifferential between the salt and aluminum layers increases. To maintainthe aluminum at 1300 F., for example, a salt layer temperature of 1400F. is often required.

In order to attain better temperature control of the salt and aluminumlayers, as well as other advantages, in the preferred embodiment asshown by Figure 4 separate heating means for the aluminum and saltlayers are used. With reference to Figure 4 the preferred embodimentconsists of a tank 38 having a lining 39 backed up by a heat insulatingmaterial 40 and adapted to hold a molten aluminum layer 41 and a moltensalt layer 42. The upper portion 43 of the lining 39, or that portion incontact with the molten salt, consists of silicon carbide while thelower portion 44 consists of any suitable refractory such as graphite orsilicate bonded alumina or silica. As in the embodiment shown in Figures1 and 2, the silicon carbide section of the liner extends slightly belowthe salt aluminum interface as shown at 45 in order to prevent possiblecontinued contact of the molten salt with the lower portion 44 of theliner.

The particular furnace shown in Figure 4, it will be noted, has anL-shaped cross-section in order to allow a larger aluminum layer depthfor the amount of aluminum used. This shape furnace having a deepportion 46 and a shallow portion 47, the cross-sectional area of thedeep portion being less than that of the shallow portion, isparticularly useful where bulky articles are to be coated or where, forexample, a precoating heat treatment requiring a large length of saltbath, through which the work piece is run before coating, is necessaryor desirable. In Figure 4 the aluminum layer is contained Within thedeep portion 46 of the tank; however, it is often desirable to have thealuminum layer extend high enough to furnish at least a thin layer onthe bottom of the shallow portion 47 of the tank in order to provideadditional aluminum layer surface for salt activation.

Electrodes 48 for resistance heating of the salt layer and impeller 54extend into the shallow portion 47 of the tank 38 from above. Theelectrodes are, of course, suitably connected to a source of electricalenergy.

To afford independent heating means for the aluminum layer an inductionheater 49, suitably connected to a source of electrical energy, ispositioned at the bottom and adjacent the floor of the deep portion 46of the tank 38. This induction heater 49 is capable of providingsuflicient heat to maintain the aluminum layer molten and attemperatures up to 1800 F. independently of the heat transferred fromthe molten salt. While only one induction heater has been shown, it isunderstood that a plurality of heaters could be used depending upon thesize of the furnace and heater and upon the amount of heat required.

The induction heating unit is positioned in a recessed portion 50 of thetank 38 to provide the necessary channeling of the aluminum forinduction heating. The width of the recessed portion 50 is suflicient toprovide the channels 51 and 52. The center of the heater 49 furnishesanother channel 53.

While I have shown the induction heating unit positioned on the bottomof the tank, it is understood of course that the heater, or heaters,could also be located on the sides of the tank in the aluminum layer.

The separate heating means for the aluminum layer is advantageous inthat it allows independent temperature control of the aluminum and saltlayers, which is not obtainable when the aluminum is heated only bycontact with the salt. Higher aluminum temperatures can be attainedwithout raising the salt layer temperature to a point where it isundesirable. It also allows for the use of a shallower salt layer thanthat required when the aluminum is heated by heat transfer from thesalt. This is desirable for the control of the rinsing action of thesalt following immersion of the work in the aluminum. The depth of themolten aluminum which can be used below the salt layer is independent ofthe temperature of the salt layer. Also, it is not necessary to restrictfurnace design to that which affords suflicient contact area between thesalt and aluminum to assure sufiicient heat transfer. It has also beenfound that the continued agitation of the aluminum and salt interfacedue to the pumping action of the aluminum induction heater increases theactivating action of the aluminum on the fluoride containing salt layer.Another advantage of the use of independent aluminum layer heating meansis that a failure of power supply to either heating circuit will notresult in a freezing of the salt and aluminum to a point where operationcannot be resumed.

The exact furnace design will of course depend upon the nature of thecoating process to be practiced. In many processes the salt layer andaluminum layer temperatures and depths required are such that noseparate heating means for the aluminum layer is required, while inothers precoating heat and cleaning treatment or postcoating rinsingwill make desirable the use of separate heaters for the aluminum. It isto be understood that the various features described in conjunction witha particular embodiment, for purposes of illustration, could be utilizedin other modifications, some of which are shown. Thus, the variousdescribed features may be interchanged in the several modificationswithout departing from the scope of the invention.

The furnaces of this invention provide eficient means for the practiceof metal coating and cleaning processes utilizing fluoride containingmolten salt as a treating agent. The cleaning and coating of steel orother metals with aluminum can be accomplished in one furnace.

It is to be understood that although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

I claim:

1. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with molten aluminum comprising a tankadapted to hold a lower layer of molten aluminum and an upper layer ofmolten salt, and said tank having a lining of silicon carbide at leastin those portions contacted by said molten salt, and a plurality ofelectrodes extending into said tank for resistance heating of said saltlayer.

2. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with a layer of molten aluminumcomprising a tank, adapted to hold a lower layer of molten aluminum andan upper layer of molten salt, a plurality of electrodes extending intosaid tank for resistance heating of said salt layer, and an impeller insaid tank for agitating said salt layer, said tank having a lining ofsilicon carbide at least in those portions contacted by said salt layer.

3. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with a layer of molten aluminum,comprising a tank adapted to hold a lower layer of molten aluminum andan upper layer of molten salt, electrodes extending into said tank fromabove and adjacent a wall of said tank for resistance heating of saidsalt layer, an impeller in said tank rotated by driving means positionedoutside said tank for agitating said salt layer, said tank having alining of silicon carbide at least in those portions contacted by saidsalt layer and of a refractory other than silicon carbide in portions incontact with said aluminum layer.

4. A furnace for coating metal with aluminum by dipping into a fluoridecontaining molten salt layer and then into a molten aluminum layercomprising a tank having a silicon carbide lining and adapted to hold amolten salt layer, a second tank submerged in said first mentioned tankfor holding molten aluminum, said second tank having silicon carbidewalls, and a series of electrodes extending into said first mentionedtank from above for resistance heating of the salt layer.

5. A furnace for coating metal with aluminum by dipping into a fluoridecontaining molten salt layer and then into a molten aluminum layercomprising a tank having a portion adjacent one side wall thereof deeperthan the other portions of said tank, said deeper portion containing amolten aluminum layer and the shallower portion of said tank containinga molten salt salt layer, said tank having a lining of silicon carbideat least in those portions contacted by said molten salt layer, and aseries of electrodes extending into said tank along one side thereof andfrom above for resistance heating of said salt layer.

6. A furnace for coating metals by dipping into a layer of fluoridecontaining molten salt and then into a layer of molten aluminumcomprising a tank adapted to contain a layer of molten aluminum and alayer of molten salt, said tank having a lining of silicon carbide atleast in those portions contacted by said molten salt, an inductionheater in said tank for heating said aluminum layer and means in saidtank, independent of said induction heater for heating said salt layer.

7. A furnace for coating metal with aluminum by dipping into a fluoridecontaining salt layer and then into a molten aluminum layer comprising atank adapted to hold a lower layer of molten aluminum and an upper layerof molten salt, said tank having a lining of silicon carbide at least inthose portions contacted by said molten salt layer, a series ofelectrodes extending into the upper portion of and fromabove said tankfor resistance heating of said salt layer and an induction heating unitin the lower portion of said tank for heating said aluminum layer.

8. A furnace for coating metal with aluminum comprising a tank havinga'deep portion adapted to hold molten aluminum and a shallow portionadapted to hold molten salt, said deep portion having a smallercrosssectional area than said shallow portion, a silicon carbide linerin said tank at least in those portions contacted by said molten salt, aseries of electrodes extending into the shallow portion of said tank forresistance heating said salt layer, and an induction heating unit in thedeep portion of said tank for heating said aluminum layer.

9. A furnace for coating metal with aluminum by dipping into a fluoridecontaining salt layer and then into a molten aluminum layer comprising atank adapted to hold a lower layer of molten aluminum and an upper layerof molten salt, said tank having a lining of silicon carbide at least inthose portions contacted by said molten salt layer, a series ofelectrodes extending into the upper portion of and from above said tankfor resistance heating of said salt layer, an induction heating unit inthe lower portion of said tank for heating said aluminum layer, and animpeller in said tank rotated by driving means positioned outside saidtank for agitating said salt layer.

10. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with molten aluminum comprising a tankadapted to hold a layer of molten aluminum and a layer of molten salt,said tank having a lining of silicon carbide in those portions contactedby said salt layer and a lining of a refractory other than siliconcarbide in those portions contacted by said aluminum layer, and means insaid furnace for heating said salt layer.

11. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with a layer of molten aluminumcomprising a tank adapted to hold a layer of molten aluminum and a layerof molten salt, means in said furnace for heating said salt layer, andan impeller in said tank for agitating said salt layer, said tank havinga lining of silicon carbide at least in those portions contacted by saidsalt layer.

12. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with a layer of molten aluminum,comprising a tank adapted to hold a lower layer of molten aluminum andan upper layer of molten salt, electrodes extending into said tank fromabove and adjacent a wall of said tank for resistance heating of saidsalt layer, an impeller in said tank rotated by driving means positionedoutside said tank for agitating said salt layer, said tank having alining resistant to corrosion by molten fluoride in those portionscontacted by said salt layer and refractory lining resistant todeterioration by molten aluminum in those portions contacted by saidaluminum layer.

13. A furnace for treating metals by dipping into a layer of fluoridecontaining molten salt in contact with molten aluminum comprising a tankadapted to hold a lower layer of molten aluminum and an upper layer ofmolten fluoride salt, said tank having a lining of silicon carbide atleast in those portions contacted by said molten salt, and heating meansin said tank for maintaining said layers in a molten state.

References Cited in the file of this patent UNITED STATES PATENTS880,743 Van Kugelgen et al Mar. 3, 1908 1,637,486 Kelleher Aug. 2, 19271,740,081 Finkbone Dec. 17, 1929 2,315,725 Moller Apr. 6,. 19432,539,215 Weil et al Jan. 23, 1951

13. A FURNACE FOR TREATING METALS BY DIPPING INTO A LAYER OF FLUORIDECONTAINING MOLTEN SALT IN CONTACT WITH MOLTEN ALUMINUM COMPRISING A TANKADAPTED TO HOLD A LOWER LAYER OF MOLTEN ALUMINUM AND AN UPPER LAYER OFMOLTEN FLUORIDE SALT, SAID TANK HAVING A LINING OF SILICON CARBIDE ATLEAST IN THOSE PORTIONS CONTACTED BY SAID MOLTEN SALT, AND HEATING MEANSIN SAID TANK FOR MAINTAINING SAID LAYERS IN A MOLTEN STATE.